How do i fold down my rear seats in my 2005 nissan sentra?

This will depend on which trim level your Sentra has. Only some models had fold down seats. Look in the Trunk. If yours has drop seats there will be two release pulls in there. They will either be a large push button or a pull lever/tag with the picture of a seat on it. They will be positioned with one in the top left corner of the seat back and one on the top right corner, usually high up.
Another quick way to check is to sit in the rear of the vehicle and look at the seat backs. Drop seats are usually split into a 2 seat back and a single seat back, (60/40 split), so there will be visible stitch lines you can slide your hand between where the joint is.

Call a Nissan dealer to ask, as some models can fold down but others cannot.

Locksmith, and AAA auto service might be needed to open it.

Several internet links, including FIXYA, suggest a knob or handle on some Nissan models on the passenger seat that may be used to fold the seat down, but not all Nissan cars have that feature, and some models may even require a key to be inserted, which would mean paying a dealer for a replacement key if they can make one for you.

Check if this video helps although it states it is for a different model. Note the 59 sec mark for the loop to pull in the front and
1.06 sec
2012 NISSAN Sentra Folding Down the Rear Seats if so equipped

I have an 87 4dr Sentra w/o folding back seats. I need to get to the upper strut bolts and they are NOT accessible from trunk. All above wrong. I have pulled up on front edge of seat with full leg and pry bar and nothing moved. I pushed back and up on seat but no luck. I pulled away carpet and there are no screws. Chilton manual says there are bolts on front but they don't exist. I looked under car and in trunk but no luck.

Also, please remove blueextc322 answer because it's for an ENGINE COMPARTMENT problem, not seats.

To fold down, follow these steps:
1. There is a loop located in the middle of the back seat - pull it forward and the bottom part of the rear seat will fold down
2. Wedge the the back seat, and it should not give you any trouble in folding down
3. To ensure that the seat goes all the way down- remove the headrest.
If your seat can not be folded-down, follow these steps to remove it:
1. Put some pressure on the bottom cushion of the back seat.
2. To release the bottom seat, pull the T handles, and then remove the cushion
3. There will be some bolts and screws on the bottom of the seat - remove them, and then you can pull out the seat.

you have to remove the bottom part of the seat and then un bolt the upper part

should be a button or lever or pull tab somewhere along the top of your back seats that allow them to fold down..... and your trunk should have an emergency release lever inside the trunk

There should be a switch somewhere along the top of the back seats that allow them to fold down....
It may be hidden or recessed into the top of the seat, or just beside the seat somewhere along the top

fold down back seat and manually release the latch them inspect lock for trunk

had to install a driver side non folding mirror, fairly simply to say the least. There's a cover on the interior of the door, near the window, remove the cover, remove the 3 screws, need i believe a 10mm socket. If a power window, unhook the circuit. Replace new mirror.